Beaver fever is caused by a parasite called giardiasis, which is found in the feces of beavers. When one drinks water that beavers swim or live in, one could ingest this nasty parasite and become infected. Beavers, however, are not the sole carriers of giardiasis. Other mammals can carry it as well - such as dogs and humans.

Drinking unfiltered water from streams, rivers or lakes where beavers live is likely the most common way of becoming infected. Although one could also become infected by touching infected feces and eating something with unwashed hands. Symptoms can range from nausea, upset stomach, abdominal cramps, diarrhea, bloating, gas, fatigue and weight loss.

Treatment is not always necessary as it can go away on its own. However, if the symptoms persist and get worse, there are treatments which can help. Certain kinds of antibiotics can work effectively to rid your body of this parasite like Metronidazole and Tinidazole. It can take anywhere from twenty-four hours to eight days to work.

More Info: en.m.wikipedia.org